Synthesis of estrogens in progenitor cells of adult fish brain: evolutive novelty or exaggeration of a more general mechanism implicating estrogens in neurogenesis?

Neuroestradiol and neuronal development: Not an exclusive male tale anymore

The brain synthesizes a variety of neurosteroids, including neuroestradiol. Inhibition of neuroestradiol synthesis results in alterations in basic neurodevelopmental processes, such as neurogenesis, neuroblast migration, neuritogenesis and synaptogenesis. Although the neurodevelopmental actions of neuroestradiol are exerted in both sexes, some of them are sex-specific, such as the well characterized effects of neuroestradiol derived from the metabolism of testicular testosterone during critical periods of male brain development. In addition, recent findings have shown sex-specific actions of neuroestradiol on neuroblast migration, neuritic growth and synaptogenesis in females. Among other factors, the epigenetic regulation exerted by X linked genes, such as Kdm6a/Utx, may determine sex-specific actions of neuroestradiol in the female brain. This review evidences the impact of neuroestradiol on brain formation in both sexes and highlights the interaction of neural steriodogenesis, hormones and sex chromosomes in sex-specific brain development.

Tissue-specific promoters regulate the transcription of cyp19a1 in the brain-pituitary-gonad axis of Japanese eel (Anguilla japonica)

Aromatase is a key enzyme that catalyzes the biosynthesis of estrogens. Previous study indicated that putative tissue-specific promoters of the one aromatase gene (cyp19a1) may drive the differential regulatory mechanisms of cyp19a1 expression in Anguilla japonica. In the present study, for elucidating the transcription characteristics and the function of putative tissue-specific promoters of cyp19a1 in the brain-pituitary-gonad (BPG) axis during vitellogenesis, we investigated the transcriptional regulation of cyp19a1 by 17β-estrogen (E2), testosterone (T), or human chorionic gonadotropin (HCG) in A. japonica. The expression of estrogen receptor (esra), androgen receptor (ara), or luteinizing hormone receptor (lhr) was up-regulated as cyp19a1 in response to E2, T, or HCG, respectively in the telencephalon, diencephalon, and pituitary. The expression of cyp19a1 was also upregulated in the ovary by HCG or T in a dose-dependent manner. Unlike in the brain and pituitary, the expression of esra and lhr, rather than ara, was upregulated by T in the ovary. Subsequently, four primary subtypes of 5'-untranslated terminal regions of cyp19a1 transcripts and the corresponding two 5' flanking regions (promoter P.I and P.II) were identified. The P.II existed in all BPG axis tissues, whereas the P.I with strong transcriptional activity was brain- and pituitary-specific. Furthermore, the transcriptional activity of promoters, the core promoter region, and the three putative hormone receptor response elements were validated. The transcriptional activity did not change when the HEK291T cells co-transfected with P.II and ar vector were exposed to T. These results suggested that the expression of cyp19a1 was upregulated indirectly through esra and lhr rather than ara by T in the ovary, whereas the expression of cyp19a1 was upregulated directly through androgen receptor and the downstream androgen response element of tissue-specific P.I in the brain and pituitary. The results of the study reveal the regulatory mechanisms of estrogen biosynthesis and provide a reference for optimizing the technology of artificially induced maturation in eels.

Pituitary luteinizing hormone synthesis starts in aromatase (cyp19a1b)-positive cells expressing esr1 and esr2b at the onset of puberty in Takifugu rubripes (fugu)

Unlike mammals, teleost fish have high aromatase activity (AA) in the pituitary. However, the cells responsible for oestradiol synthesis and the local physiological roles of this hormone remain unclear. Hence, we investigated the effects of age and development on steroidogenic activity, mRNA expression, and cyp19a1b localization in the pituitary gland of the Japanese pufferfish Takifugu rubripes. Under aquaculture conditions, AA was highest after puberty, and the mRNA expression levels of cyp19a1b and the oestrogen receptors esr1 and 2b and the level of serum testosterone (T) were significantly increased after puberty compared with the other developmental stages in male and female pufferfish. Immunohistochemistry using multiple antibodies and in situ hybridization analysis revealed that Cyp19a1b colocalizes with luteinizing hormone (LH) in pituitary cells. Furthermore, Esr1 was localized in the nuclei of all hormone-producing cells, whereas Esr2b was localized only in the nuclei of Cyp19- and LH-positive cells. The administration of an aromatizable androgen (T) or oestrogen (E2) to reproductively inactive females induced LH synthesis in vivo. We prepared spheroids from pituitary cells to investigate the role of local E2 in LH synthesis in vitro. Immunohistochemical analysis of spheroids showed that T-induced LH synthesis could be blocked by an aromatase inhibitor and/or an ER antagonist but not an AR antagonist. Taken together, these findings suggest that LH synthesis is initiated in cyp19a1b-, esr1-, and esr2b-expressing cells at the onset of puberty under the control of steroidal feedback, and both feedback and local oestrogen may be involved in controlling LH synthesis in these cells.

Mitochondria of teleost radial glia: A novel target of neuroendocrine disruption by environmental chemicals?

In teleost fish, radial glial cells (RGCs) are progenitor cells for neurons and the major cell type synthesizing neuroestrogens. We hypothesized that chemical exposure impairs mitochondrial bioenergetics of RGCs, which then may lead to downstream consequences for neuroestrogen production. Here we provide proof of concept that mitochondria of RGCs can be perturbed by fungicides. We isolated RGCs from a mixed sex population of goldfish (Carassius auratus) and measured metabolic capacity of primary cells to a model mitotoxin fluazinam, a broad-spectrum fungicide that inhibits mitochondria electron transport chain (or ETC) Complex I. Using immunocytochemistry and real-time PCR, we demonstrate that the goldfish primary cell cultures are highly enriched for glia after multiple passages. Cytotoxicity assays revealed that glia treated with >25 μM fluazinam for 24 and 48-h showed reduced viability. As such, metabolic assays were conducted with non-cytotoxic concentrations (0.25-12.5 μM). Fluazinam did not affect oxygen consumption rates of RGCs at 24 h, but after 48 h, oligomycin induced ATP-linked respiration was decreased by both 6.25 and 12.5 μM fluazinam. Moreover, concentrations as low as 0.25 μM disrupted the mitochondrial membrane potential of RGCs, reflecting strong uncoupling effects of the fungicide on mitochondria. Here we provide proof of concept that mitochondrial bioenergetics of teleostean RGCs can be responsive to agrochemicals. Additional studies are required to address low-dose exposures in vivo and to determine if metabolic disruption impairs neuroendocrine functions of RGCs. We propose this mechanism constitutes a novel aspect of neuroendocrine disruption, significant because dysregulation of neuron-glia communication is expected to contribute to neuroendocrine disruption.

Zebrafish Optomotor Response and Morphology Are Altered by Transient, Developmental Exposure to Bisphenol-A

Estrogen-specific endocrine disrupting compounds (EDCs) are potent modulators of neural and visual development and common environmental contaminants. Using zebrafish, we examined the long-term impact of abnormal estrogenic signaling by testing the effects of acute, early exposure to bisphenol-A (BPA), a weak estrogen agonist, on later visually guided behaviors. Zebrafish aged 24 h postfertilization (hpf), 72 hpf, and 7 days postfertilization (dpf) were exposed to 0.001 μM or 0.1 μM BPA for 24 h, and then allowed to recover for 1 or 2 weeks. Morphology and optomotor responses (OMRs) were assessed after 1 and 2 weeks of recovery for 24 hpf and 72 hpf exposure groups; 7 dpf exposure groups were additionally assessed immediately after exposure. Increased notochord length was seen in 0.001 μM exposed larvae and decreased in 0.1 μM exposed larvae across all age groups. Positive OMR was significantly increased at 1 and 2 weeks post-exposure in larvae exposed to 0.1 μM BPA when they were 72 hpf or 7 dpf, while positive OMR was increased after 2 weeks of recovery in larvae exposed to 0.001 μM BPA at 72 hpf. A time-delayed increase in eye diameter occurred in both BPA treatment groups at 72 hpf exposure; while a transient increase occurred in 7 dpf larvae exposed to 0.1 μM BPA. Overall, short-term developmental exposure to environmentally relevant BPA levels caused concentration- and age-dependent effects on zebrafish visual anatomy and function.

Plasticity of Lh cells caused by cell proliferation and recruitment of existing cells

Luteinizing hormone (Lh) and follicle-stimulating hormone (Fsh) control reproduction in vertebrates. Using a transgenic line of medaka, in which green fluorescent protein expression is controlled by the endogenous lhb promotor, we studied development and plasticity of Lh cells, comparing juveniles and adults of both genders. Confocal imaging and 3D reconstruction revealed hypertrophy and hyperplasia of Lh cells in both genders from juvenile to adult stages. We show that Lh cell hyperplasia may be caused by recruitment of existing pituitary cells that start to produce lhb, as evidenced by time lapse recordings of primary pituitary cell cultures, and/or through Lh cell proliferation, demonstrated through a combination of 5-bromo-2'-deoxyuridine incubation experiments and proliferating cell nuclear antigen staining. Proliferating Lh cells do not belong to the classical type of multipotent stem cells, as they do not stain with anti-sox2. Estradiol exposure in vivo increased pituitary cell proliferation, particularly Lh cells, whereas pituitary lhb and gpa expression levels decreased. RNA-seq and in situ hybridization showed that Lh cells express two estrogen receptors, esr1 and esr2b, and the aromatase gene cyp19a1b, suggesting a direct effect of estradiol, and possibly androgens, on Lh cell proliferation. In conclusion, our study reveals a high degree of plasticity in the medaka Lh cell population, resulting from a combination of recruitment and cell proliferation.

Non-reproductive Functions of Aromatase in the Central Nervous System Under Physiological and Pathological Conditions

The modulation of brain function and behavior by steroid hormones was classically associated with their secretion by peripheral endocrine glands. The discovery that the brain expresses the enzyme aromatase, which produces estradiol from testosterone, expanded this traditional concept. One of the best-studied roles of brain estradiol synthesis is the control of reproductive behavior. In addition, there is increasing evidence that estradiol from neural origin is also involved in a variety of non-reproductive functions. These include the regulation of neurogenesis, neuronal development, synaptic transmission, and plasticity in brain regions not directly related with the control of reproduction. Central aromatase is also involved in the modulation of cognition, mood, and non-reproductive behaviors. Furthermore, under pathological conditions aromatase is upregulated in the central nervous system. This upregulation represents a neuroprotective and likely also a reparative response by increasing local estradiol levels in order to maintain the homeostasis of the neural tissue. In this paper, we review the non-reproductive functions of neural aromatase and neural-derived estradiol under physiological and pathological conditions. We also consider the existence of sex differences in the role of the enzyme in both contexts.

Steroid Transport, Local Synthesis, and Signaling within the Brain: Roles in Neurogenesis, Neuroprotection, and Sexual Behaviors

Sex steroid hormones are synthesized from cholesterol and exert pleiotropic effects notably in the central nervous system. Pioneering studies from Baulieu and colleagues have suggested that steroids are also locally-synthesized in the brain. Such steroids, called neurosteroids, can rapidly modulate neuronal excitability and functions, brain plasticity, and behavior. Accumulating data obtained on a wide variety of species demonstrate that neurosteroidogenesis is an evolutionary conserved feature across fish, birds, and mammals. In this review, we will first document neurosteroidogenesis and steroid signaling for estrogens, progestagens, and androgens in the brain of teleost fish, birds, and mammals. We will next consider the effects of sex steroids in homeostatic and regenerative neurogenesis, in neuroprotection, and in sexual behaviors. In a last part, we will discuss the transport of steroids and lipoproteins from the periphery within the brain (and vice-versa) and document their effects on the blood-brain barrier (BBB) permeability and on neuroprotection. We will emphasize the potential interaction between lipoproteins and sex steroids, addressing the beneficial effects of steroids and lipoproteins, particularly HDL-cholesterol, against the breakdown of the BBB reported to occur during brain ischemic stroke. We will consequently highlight the potential anti-inflammatory, anti-oxidant, and neuroprotective properties of sex steroid and lipoproteins, these latest improving cholesterol and steroid ester transport within the brain after insults.

Voluntary exercise increases adult hippocampal neurogenesis by increasing GSK-3β activity in mice

Exercise has been proven to promote learning and memory, and is closely related to increased adult neurogenesis in the hippocampus. In our study, the β subunit of Glycogen synthase kinase-3 (GSK3β) can be significantly regulated by exercise, and the modulation of GSK3β activity can enhance adult neurogenesis and memory. To explore the mechanism by which exercise can improve cognitive function and adult neurogenesis, and the role GSK3β plays in this process, we established a mouse model of voluntary exercise to examine the expression and activity of GSK3β, and its associated signaling pathways, in the hippocampus dentate gyrus. The results showed an obvious increase in adult neurogenesis and cognitive functions, and the up-regulation of GSK3β, after exercise. The activity of the insulin pathway, which negatively regulates GSK3β, was also increased. Moreover, our results showed that the dopamine D₁ receptor (DARP D₁) pathway and adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) were also activated, which indicates a relationship between GSK3β and neurogenesis. Overall, our findings demonstrated that voluntary exercise promotes cognition and neurogenesis in the adult mouse dentate gyrus by the regulation of GSK3β expression and activity, which may be implemented through the DARP D₁ receptor-signaling pathway.

Association between apoptotic neural tissue and cell proliferation in the adult teleost brain

Injury to neuronal tissues in the central nervous system (CNS) of mammals results in neural degeneration and sometime leads to loss of function, whereas fish retain a remarkable potential for neuro-regeneration throughout life. Thus, understanding the mechanism of neuro-regeneration in fish CNS would be useful to improve the poor neuro-regenerative capability in mammals. In the present study, we characterized a neuro-regenerative process in the brain of a cichlid, tilapia, Oreochromis niloticus. Morphological observations showed that the damaged brain region (habenula) successfully regrew and reinnervated axonal projections by 60 days post-damage. A fluorescent carbocyanine tracer, DiI tracing revealed a recovery of the major neuronal projection from the regenerated habenula to the interpenduncular nucleus by 60 days post-damage. TUNEL assay showed a significant increase of apoptotic cells (~234%, P<0.01) at one day post-damage, while the number of bromodeoxyuridine (BrdU)-positive proliferative cells were significantly increased (~92%, P<0.05) at 7 days post-damage compared with sham-control fish. To demonstrate a potential role of apoptotic activity in the neuro-regeneration, effects of degenerative neural tissue on cell proliferation were examined in vivo. Implantation of detached neural but not non-neural tissues into the cranial cavity significantly (P<0.01) increased the number of BrdU-positive cells nearby the implantation regions at 3 days after the implantation. Furthermore, local injection of the protein extract and cerebrospinal fluid collected from injured fish brain significantly induced cell proliferation in the brain. These results suggest that factor(s) derived from apoptotic neural cells may play a critical role in the neuro-regeneration in teleost brain.

Estrogen Receptor α Regulates β-Cell Formation During Pancreas Development and Following Injury

Identifying pathways for β-cell generation is essential for cell therapy in diabetes. We investigated the potential of 17β-estradiol (E2) and estrogen receptor (ER) signaling for stimulating β-cell generation during embryonic development and in the severely injured adult pancreas. E2 concentration, ER activity, and number of ERα transcripts were enhanced in the pancreas injured by partial duct ligation (PDL) along with nuclear localization of ERα in β-cells. PDL-induced proliferation of β-cells depended on aromatase activity. The activation of Neurogenin3 (Ngn3) gene expression and β-cell growth in PDL pancreas were impaired when ERα was turned off chemically or genetically (ERα(-/-)), whereas in situ delivery of E2 promoted β-cell formation. In the embryonic pancreas, β-cell replication, number of Ngn3(+) progenitor cells, and expression of key transcription factors of the endocrine lineage were decreased by ERα inactivation. The current study reveals that E2 and ERα signaling can drive β-cell replication and formation in mouse pancreas.

Molecular cloning, characterization and expression of Lc-Sox11a in large yellow croaker Larimichthys crocea

Sox genes play important roles in various developmental processes such as sex determination, embryogenesis, oogenesis, neurogenesis, and larval development. In order to clarify the roles of Sox genes in the developmental process of large yellow croaker, the full-length cDNA of the Sox11a gene (Lc-Sox11a) was cloned for the first time. Bioinformatics analysis indicated that Lc-Sox11a contains a protein of 366 amino acids with a Ser-rich region, a C-terminal conserved region, and a high mobility group box. The expression of Lc-Sox11a in different tissues of both sexes and in different developmental embryonic stages revealed that Lc-Sox11a were expressed with tissue and gender specificity, of which the expression level in female was ovary>brain>eye>gill; in male was brain>testis>gill. The gender differences occurred in the brain and eye with the male brain>female brain, female eye>male eye. Moreover, the expression of Lc-Sox11a in the gonad and brain at different growth stages was detected. Significant up-regulated expression of Lc-Sox11a was found in the ovary and the male brain at 1000dph (days post hatching) compared with 270dph and 635dph. However, significant down-regulated expression of Lc-Sox11a occurred in the testis with growth. Besides, the expression of Lc-Sox11a in the female brain showed a trend of first rising then falling, with the highest peak in 635dph. The results of in situ hybridization displayed that Lc-Sox11a was widely distributed only in cytoplasm of oocytes at each stage in oogenesis. In early stage of oocytes, Lc-Sox11a was expressed weakly and evenly. As the appearance of vacuoles and synthesis of yolks, positive signals of Lc-Sox11a distributed intensively in the residual cytoplasm. In spermatogenesis, Lc-Sox11a was distributed in cytoplasm of all male germ cells except spermatozoon with spermatogonium>spermatocyte>spermatid. During embryogenesis, Lc-Sox11a was expressed in most embryonic stages, the highest expression occurred in the formation-of-eye-lens stage, closely followed by the closure-of-blastopore stage, then the beginning-of-heart-pulsation stage. The results of whole mount in situ hybridization showed that the expression of Lc-Sox11a began to increase beginning with the multiple-cell stage, with the major distribution of Lc-Sox11a in the brain and eye areas in the pre-hatching stage.

Activation of brain steroidogenesis and neurogenesis during the gonadal differentiation in protandrous black porgy, Acanthopagrus schlegelii

The early brain development, at the time of gonadal differentiation was investigated using a protandrous teleost, black porgy. This natural model of monosex juvenile fish avoids the potential complexity of sexual dimorphism. Brain neurogenesis was evaluated by histological analyses of the diencephalon, at the time of testicular differentiation (in fish between 90 and 150 days after hatching). Increases in the number of both Nissl-stained total brain cells, and Pcna-immunostained proliferative brain cells were observed in specific area of the diencephalon, such as ventromedialis thalami and posterior preoptic area, revealing brain cell proliferation. qPCR analyses showed significantly higher expression of the radial glial cell marker blbp and neuron marker bdnf. Strong immunohistochemical staining of Blbp and extended cellular projections were observed. A peak expression of aromatase (cyp19a1b), as well as an increase in estradiol (E2 ) content were also detected in the early brain. These data demonstrate that during gonadal differentiation, the early brain exhibits increased E2 synthesis, cell proliferation, and neurogenesis. To investigate the role of E2 in early brain, undifferentiated fish were treated with E2 or aromatase inhibitor (AI). E2 treatment upregulated brain cyp19a1b and blbp expression, and enhanced brain cell proliferation. Conversely, AI reduced brain cell proliferation. Castration experiment did not influence the brain gene expression patterns and the brain cell number. Our data clearly support E2 biosynthesis in the early brain, and that brain E2 induces neurogenesis. These peak activity patterns in the early brain occur at the time of gonad differentiation but are independent of the gonads.

Genes encoding aromatases in teleosts: evolution and expression regulation

Cytochrome P450 aromatases, encoded by cyp19a1 genes, catalyzes the conversion of androgens to estrogens and plays important roles in the reproduction of vertebrates. Vertebrate cyp19a1 genes showed high synteny in chromosomal locations and conservation in sequences during evolution. However, amphioxus cyp19a1 does not show synteny to vertebrate cyp19a1. Teleost fish possess two copies of the cyp19a1 gene, which were postulated to result from a fish-specific genome duplication. The duplicated copies of fish cyp19a1 genes evolved into the brain and ovarian forms of cytochrome P450 aromatase genes, cyp19a1a and cyp19a1b, respectively, with different regulatory mechanisms of expression, through subfunctionalization under long-term selective pressure. In addition to the estradiol (E2) auto-regulatory loop, there may be other mechanisms responsible for the high expression of aromatase in the teleost brain. The study of the two cyp19a1 copies in teleost fish will shed light on the general evolution, function, and regulation of vertebrate cyp19a1.

Modulation of aromatase activity as a mode of action for endocrine disrupting chemicals in a marine fish

The steroidogenic enzyme aromatase catalyzes the conversion of androgens to estrogens and therefore plays a central role in reproduction. In contrast to most vertebrates, teleost fish have two distinct forms of aromatase. Because brain aromatase activity in fish is up to 1000 times that in mammals, fish may be especially susceptible to negative effects from environmental endocrine-disrupting chemicals (EDCs) that impact aromatase activity. In this study, the effects of estradiol (E2), ethynylestradiol (EE2), octylphenol (OP), and androstatrienedione (ATD) on reproduction and aromatase activity in brains and gonads from the marine fish cunner (Tautogolabrus adspersus) was investigated. The purpose of the study was to explore the relationship between changes in aromatase activity and reproductive output in a marine fish, as well as compare aromatase activity to two commonly used indicators of EDC exposure, plasma vitellogenin (VTG) and gonadosomatic index (GSI). Results with E2, EE2, and ATD indicate that aromatase activity in cunner brain and ovary are affected differently by exposure to these EDCs. In the case of E2 and EE2, male brain aromatase activity was signficantly increased by these treatments, female brain aromatase activity was unaffected, and ovarian aromatase activity was significantly decreased. Treatment with the aromatase inhibitor ATD resulted in significantly decreased aromatase activity in male and female brain, but had no significant impact on ovarian aromatase activity. Regardless of test chemical, a decrease or an increase in male brain aromatase activity relative to controls was associated with decreased egg production in cunner and was also correlated with significant changes in GSI in both sexes. E2 and EE2 significantly elevated plasma VTG in males and females, while ATD had no significant effect. Treatment of cunner with OP had no significant effect on any measured endpoint. Overall, results with these exposures indicate EDCs that impact aromatase activity also affect reproductive output in spawning cunner.

Sex-different effects of tributyltin on brain aromatase, estrogen receptor and retinoid X receptor gene expression in rockfish (Sebastiscus marmoratus)

Since the brain plays important roles in reproduction, the brain aromatase (Cyp19b), estrogen receptor (ER), retinoid X receptor (RXR) α and peroxisome proliferator-activated receptor γ were examined in rockfish after TBT exposure (1, 10, and 100 ng L(-1)). The results showed that the Cyp19b expression was elevated in the male rockfish, while no effect was produced in the females. Inconsistently, serum testosterone and 17β-estradiol showed no change in the males, while an increase of testosterone and a decrease of 17β-estradiol were observed in the females. TBT affected the ER expression in the males depending on the concentrations, however, no change was observed in the females. In addition, TBT elevated the RXRα expression in the males but produced an opposite effect in the females. In conclusion, TBT might have had sex-different effects on the brain Cyp19b, ER and RXR expression in rockfish, indicating a complex endocrine disrupting effect of TBT.

Expression of aromatase in radial glial cells in the brain of the Japanese eel provides insight into the evolution of the cyp191a gene in Actinopterygians

The cyp19a1 gene that encodes aromatase, the only enzyme permitting conversion of C19 aromatizable androgens into estrogens, is present as a single copy in the genome of most vertebrate species, except in teleosts in which it has been duplicated. This study aimed at investigating the brain expression of a cyp19a1 gene expressed in both gonad and brain of Japanese eel, a basal teleost. By means of immunohistochemistry and in situ hybridization, we show that cyp19a1 is expressed only in radial glial cells of the brain and in pituitary cells. Treatments with salmon pituitary homogenates (female) or human chorionic gonadotrophin (male), known to turn on steroid production in immature eels, strongly stimulated cyp19a1 messenger and protein expression in radial glial cells and pituitary cells. Using double staining studies, we also showed that aromatase-expressing radial glial cells exhibit proliferative activity in both the brain and the pituitary. Altogether, these data indicate that brain and pituitary expression of Japanese eel cyp19a1 exhibits characteristics similar to those reported for the brain specific cyp19a1b gene in teleosts having duplicated cyp19a1 genes. This supports the hypothesis that, despite the fact that eels also underwent the teleost specific genome duplication, they have a single cyp19a1 expressed in both brain and gonad. Such data also suggest that the intriguing features of brain aromatase expression in teleost fishes were not gained after the whole genome duplication and may reflect properties of the cyp19a1 gene of ancestral Actinopterygians.

Androgen rather than estrogen up-regulates brain-type cytochrome P450 aromatase (cyp19a1b) gene via tissue-specific promoters in the hermaphrodite teleost ricefield eel Monopterus albus

CYP19A1 in the brain and pituitary of vertebrates is important for reproductive and non-reproductive processes. In teleosts, it is broadly accepted that estradiol (E(2)) up-regulates cyp19a1b gene via a positive autoregulatory loop. Our present study, however, showed that E(2) did not up-regulate ricefield eel cyp19a1b in the hypothalamus and pituitary, whereas dihydrotestosterone (DHT) or testosterone (T) stimulated cyp19a1b expression only in the pituitary. Two tissue-specific promoters, namely promoter I and II directing the expression in the brain and pituitary respectively, were identified. Promoter I contained a non-consensus estrogen response element (ERE), and consequently did not respond to E(2). Promoter II contained an androgen response element (ARE) and consequently responded to DHT. Taken together, these results demonstrated a novel steroidal regulation of cyp19a1b gene expression and an alternative usage of tissue-specific cyp19a1b promoters in the brain and pituitary of a teleost species, the ricefield eel.

Estrogen regulation of brain vasotocin secretion in the catfish Heteropneustes fossilis: an interaction with catecholaminergic system

Vasotocin (VT) is a basic neurohypophysial nonapeptide in non-mammalian vertebrates and is involved in diverse functions like osmoregulation, reproduction, metabolism and behavior. In this study, we report that estradiol-17β (E(2)) regulates brain and plasma VT secretion through the involvement of the catecholaminergic (CA) system. To demonstrate this, E(2) level was altered through ovariectomy (OVX, 3 weeks) and replacement study with low and high E(2) doses (0.1 and 0.5 μg/g body weight). CA activity was inhibited by treatment with α-methylparatyrosine (α-MPT; 250 μg/g body weight), a competitive inhibitor of tyrosine hydroxylase. VT was assayed by an enzyme immunoassay method. In the sham group, the low E(2) dose produced 82% and 104% increase, respectively, in brain and plasma VT levels. The high E(2) dose decreased the VT levels significantly. The low E(2) dose decreased brain E(2) but elevated plasma E(2). In the high E(2) group, the E(2) level increased further in both brain and plasma. OVX resulted in a significant inhibition (69% and 25%, respectively) of both brain and plasma VT, which was correlated with low E(2) levels. The low E(2) dose not only reversed the inhibition, but increased the VT level in both brain and plasma in comparison to the sham groups. The high E(2) replacement inhibited VT levels further low in both brain and plasma. The α-MPT treatment inhibited VT levels significantly in both sham and OVX groups. The drug treatment abolished partially the restorative effect of the low E(2) dose in the ovariectomized fish. In the high E(2) dose group, α-MPT decreased brain and plasma VT levels further low compared to the sham + 0. 5 μg E(2) group or OVX + 0.5 μg E(2) group except the brain VT level, which increased in the OVX+0.5 μg E(2) group. It is inferred that E(2) may exert biphasic effects on VT through the mediation of the CA system.

The brain of teleost fish, a source, and a target of sexual steroids

Neurosteroids are defined as steroids de novo synthesized in the central nervous system. While the production of neurosteroids is well documented in mammals and amphibians, there is less information about teleosts, the largest group of fish. Teleosts have long been known for their high brain aromatase and 5α-reductase activities, but recent data now document the capacity of the fish brain to produce a large variety of sex steroids. This article aims at reviewing the available information regarding expression and/or activity of the main steroidogenic enzymes in the brain of fish. In addition, the distribution of estrogen, androgen, and progesterone nuclear receptors is documented in relation with the potential sites of production of neurosteroids. Interestingly, radial glial cells acting as neuronal progenitors, appear to be a potential source of neurosteroids, but also a target for centrally and/or peripherally produced steroids.

Nuclear progesterone receptors are up-regulated by estrogens in neurons and radial glial progenitors in the brain of zebrafish

In rodents, there is increasing evidence that nuclear progesterone receptors are transiently expressed in many regions of the developing brain, notably outside the hypothalamus. This suggests that progesterone and/or its metabolites could be involved in functions not related to reproduction, particularly in neurodevelopment. In this context, the adult fish brain is of particular interest, as it exhibits constant growth and high neurogenic activity that is supported by radial glia progenitors. However, although synthesis of neuroprogestagens has been documented recently in the brain of zebrafish, information on the presence of progesterone receptors is very limited. In zebrafish, a single nuclear progesterone receptor (pgr) has been cloned and characterized. Here, we demonstrate that this pgr is widely distributed in all regions of the zebrafish brain. Interestingly, we show that Pgr is strongly expressed in radial glial cells and more weakly in neurons. Finally, we present evidence, based on quantitative PCR and immunohistochemistry, that nuclear progesterone receptor mRNA and proteins are upregulated by estrogens in the brain of adult zebrafish. These data document for the first time the finding that radial glial cells are preferential targets for peripheral progestagens and/or neuroprogestagens. Given the crucial roles of radial glial cells in adult neurogenesis, the potential effects of progestagens on their activity and the fate of daughter cells require thorough investigation.

Developmental expression of genes involved in neural estrogen biosynthesis and signaling in the brain of the orange-spotted grouper Epinephelus coioides during gonadal sex differentiation

In the brain, the synthesis of neurosteroids and receptor activation during gonadal sex differentiation in teleosts are poorly understood. In the present study, the protogynous orange-spotted grouper (Epinephelus coioides) was selected as a model fish, and we hypothesized that de novo synthesis of neural estrogen may play a role in the female grouper brain during gonadal sex differentiation. We investigated the temporal expression of the genes StAR, cyp19a1b and pcna and the sex steroid nuclear receptors for estrogen (ERα, ERβ1 and ERβ2), androgen (AR) and the plasma membrane-associated estrogen receptor (GPR30) in the brain during early developmental ages from 90 days after hatching (dah) to 180dah after gonadal sex differentiation. Our results revealed that mRNA for ERs and GPR30 but not AR was significantly increased at 110dah (a time close to gonadal sex differentiation) in the forebrain and midbrain and for cyp19a1b at 110dah in the forebrain. Brain aromatase activity and estradiol (E2) levels, but not testosterone (T), were increased in the forebrain at 110 and 120dah, respectively. Furthermore, exogenous E2 stimulated cyp19a1b transcripts in the forebrain and hypothalamus and immunoreactive (ir)Cyp19a1b (aromatase enzyme) in the forebrain. irCyp19a1b localized in the glial cells of the forebrain regions. Therefore, we identified a peak of functional aromatase activity and estrogen signaling in the early grouper brain during gonadal sex differentiation. Moreover, pcna transcripts (a marker for cell proliferation activity) were higher in the early brain at 110-150dah. Thus, a peak time of development in the brain is suggested to occur during gonadal sex differentiation in the grouper.

17α-Ethinylestradiol and nonylphenol affect the development of forebrain GnRH neurons through an estrogen receptors-dependent pathway

There is growing evidence that neuroendocrine circuits controlling development and reproduction are targeted by EDCs. We have previously demonstrated that low concentrations of 17α-ethinylestradiol (EE2) disrupt the development of forebrain GnRH neurons during zebrafish development. The objectives of the present study were to determine whether the weak estrogenic compound, nonylphenol (NP), could elicit similar effects to EE2 and to what extent the estrogen receptors are involved in mediating these effects. Using immunohistochemistry, we confirmed that EE2 exposure induces an increase in the number of GnRH-ir neurons and we demonstrated that NP is able to produce similar effects in a concentration-dependent manner. The effects of both NP and EE2 were shown to be blocked by the estrogen receptors (ERs) antagonist ICI 182-780, demonstrating the involvement of functional ERs in mediating their effects. Altogether, these results highlight the need to consider neuroendocrine networks as critical endpoints in the field of endocrine disruption.

Sexual dimorphism in the brain aromatase expression and activity, and in the central expression of other steroidogenic enzymes during the period of sex differentiation in monosex rainbow trout populations

Using genetic monosex male and female rainbow trout populations, the potential sex differences in the central expression of estrogen receptors (esr1, esr2a, esr2b), brain aromatase (cyp19a1b) and some other steroidogenic enzymes was studied over the period of sex differentiation (from 35 to 63 dpf: days post-fertilization) using quantitative polymerase chain reaction (q-PCR). In addition, aromatase activity was evaluated during this period. The results indicated that brain aromatase (cyp19a1b) expression and activity showed a clear and significant sexually dimorphic pattern with higher levels in male brain between 35 and 53 dpf before the time of gonad morphological differentiation. At that time the expression of a key enzyme involved in the conversion of cholesterol into steroids, the cyp11a1 (p450scc), as well as the estrogen receptors were also sexually dimorphic. The dimorphism was lost from 56 dpf onwards. Transcription factors such as nr5a1b (sf1) and nr0b1 (dax1), but not foxl2a were also higher in males than in females. These results demonstrate that, before or during the early period of morphological gonad differentiation, the brain exhibits a clear sexual dimorphism with respect to the expression and activity of aromatase as well as of certain enzymes and factors involved in steroid synthesis as p450scc and sf1. The results suggest a higher potentiality to produce estrogens by male brains during sex differentiation time.

Estradiol meets notch signaling in developing neurons

The transmembrane receptor Notch, a master developmental regulator, controls gliogenesis, neurogenesis, and neurite development in the nervous system. Estradiol, acting as a hormonal signal or as a neurosteroid, also regulates these developmental processes. Here we review recent evidence indicating that estradiol and Notch signaling interact in developing hippocampal neurons by a mechanism involving the putative membrane receptor G protein-coupled receptor 30. This interaction is relevant for the control of neuronal differentiation, since the downregulation of Notch signaling by estradiol results in the upregulation of neurogenin 3, which in turn promotes dendritogenesis.

Neuroendocrine effects of endocrine disruptors in teleost fish

Because a large proportion of potential endocrine disruptors (EDC) end up in surface waters, aquatic species are particularly vulnerable to their potential adverse effects. Recent studies identified a number of brain targets for EDC commonly present in environmentally relevant concentrations in surface waters. Among those neuronal systems disrupted by EDC are the gonadotropin-releasing hormone (GnRH) neurons, the dopaminergic and serotoninergic circuits, and more recently the Kiss/GPR54 system, which regulates gonadotropin release. However, one of the most striking effects of EDC, notably estrogen mimics, is their impact on the cyp19a1b gene that encodes the brain aromatase isoform in fish. Moreover, this is the only example in which the molecular basis of endocrine disruption is fully understood. The aims of this review were to (1) synthesize the most recent discoveries concerning the EDC effects upon neuroendocrine systems of fish and (2) provide, when possible, the underlying molecular basis of disruption for each system concerned. The potential adverse effects of EDC on neurogenesis, puberty, and brain sexualization are also described. It is important to point out the future environmental, social, and economical issues arising from endocrine disruption studies in the context of risk assessment.

Transcriptional activity and biological effects of mammalian estrogen receptor ligands on three hepatic estrogen receptors in Mozambique tilapia

Like other fish species, Mozambique tilapia has three forms of estrogen receptor, ERα, ERβ1, and ERβ2. A primary function of 17β-estradiol (E(2)) in oviparous species is the hepatic induction of the yolk precursor protein, vitellogenin (Vg). To characterize the roles of ERs in Vg production, transactivation assays and an in vivo study were carried out utilizing agonists for mammalian ERα and ERβ, and an antagonist for mammalian ERα, propyl-pyrazole-triol (PPT), diarylpropionitrile (DPN), and methyl-piperidino-pyrazole (MPP), respectively. ERα was more sensitive and responsive to PPT than ERβ1 or ERβ2 in transactivation assays. All ER isoforms indicated equivalent responsiveness to DPN compared with E(2), although sensitivity to DPN was lower. MPP exhibited antagonistic action on transactivation of all ER isoforms and reduced the E(2) effect on Vg and ERα 48h post-injection. DPN increased ERα and Vg expression and plasma Vg post-injection, whereas PPT was without effect; DPN seems to stimulate Vg production through activation of ERα. The ligand binding domain of all tilapia ER forms shares only 60-65% amino acid identity with human ERα and ERβ. This, together with our results, clearly indicates that agonistic or antagonistic characteristics of PPT, DPN and MPP cannot be extrapolated from mammalian to piscine ERs.

A second form of Sox11 homologue identified in the orange-spotted grouper Epinephelus coioides: analysis of sequence and mRNA expression patterns

Sox genes, a family of genes related to the mammalian sex-determining region Y (SRY) gene, are found throughout the animal kingdom, and involved in diverse developmental processes including sex determination and neurogenesis. Previously, we have identified one sox11 homologue, sox11b, from the ovary of the orange-spotted grouper. In the present study, another sox11 homologue, sox11a, was cloned from the brain. The orange-spotted grouper Sox11a contained the signature features of mammalian SOX11 homologues except the Pro-Glu rich region, was clustered with Sox11a homologues of other teleosts in the phylogenetic tree, and shared higher homologies with Sox11 of other species than the duplicated copy Sox11b. Interestingly, significant conservation was observed in the 3'UTR of sox11a but not sox11b transcripts when compared with mammalian Sox11 homologues. The expression of sox11a mRNA was detected in a wide range of tissues, with higher abundances in the central nervous system. During embryogenesis and larval development, the expression of sox11a mRNA remained at considerably high levels at all stages examined, from newly fertilized eggs, through organogenesis, to the larvae 18days posthatching. Together, these results indicated that the orange-spotted grouper sox11a was evolutionarily more conserved than sox11b, and may play important roles in neurogenesis, embryogenesis, and larval development.

Environmentally relevant exposure to 17alpha-ethinylestradiol affects the telencephalic proteome of male fathead minnows

Estrogens are key mediators of neuronal processes in vertebrates. As such, xenoestrogens present in the environment have the potential to alter normal central nervous system (CNS) function. The objectives of the present study were (1) to identify proteins with altered abundance in the male fathead minnow telencephalon as a result of low-level exposure to 17alpha-ethinylestradiol (EE(2)), and (2) to better understand the underlying mechanisms of 17beta-estradiol (E(2)) feedback in this important neuroendocrine tissue. Male fathead minnows exposed to a measured concentration of 5.4 ng EE(2)/L for 48 h showed decreased plasma E(2) levels of approximately 2-fold. Of 77 proteins that were quantified statistically, 14 proteins were down-regulated after EE(2) exposure, including four histone proteins, ATP synthase, H+ transporting subunits, and metabolic proteins (lactate dehydrogenase B4, malate dehydrogenase 1b). Twelve proteins were significantly induced by EE(2) including microtubule-associated protein tau (Mapt), astrocytic phosphoprotein, ependymin precursor, and calmodulin. Mapt showed an increase in protein abundance but a decrease in mRNA expression after EE(2) exposure(,) suggesting there may be a negative feedback response in the telencephalon to decreased mRNA transcription with increasing Mapt protein abundance. These results demonstrate that a low, environmentally relevant exposure to EE(2) can rapidly alter the abundance of proteins involved in cell differentiation and proliferation, neuron network morphology, and long-term synaptic potentiation. Together, these findings provide a better understanding of the molecular responses underlying E(2) feedback in the brain and demonstrate that quantitative proteomics can be successfully used in ecotoxicology to characterize affected cellular pathways and endocrine physiology.

Effects on guppy brain aromatase activity following short-term steroid and 4-nonylphenol exposures

Brain estrogen production, performed by the enzyme aromatase, can be disrupted/affected in teleost fish exposed to endocrine disruptors found in polluted aquatic environments. The guppy (Poecilia reticulata) was previously studied and confirmed to suffer negative effects on reproductive behaviors following inhibition of the brain aromatase reaction. Here adult guppies (Poecilia reticulata) of both genders were subjected to known endocrine disruptors: the androgen androstenedione (A), the synthetic estrogen 17alpha-ethinylestradiol (EE(2)), and the estrogenic surfactant 4-nonylphenol (NP), at high (50 microg/L) and at environmentally relevant concentrations (10 ng/L EE(2), 5 microg/L NP, and 0.7 microg/L A) for 2 weeks followed by measurements of brain aromatase activity (bAA). In the adult males, bAA was stimulated by A and EE(2) at 50 microg/L. Female activity was also stimulated by the higher estrogenic treatment. At environmentally relevant concentrations only the EE(2) treatment affected bAA, and only in males. The alkylphenolic substance NP produced no effect in either of the experiments, not on males nor females. The results indicate that short-term steroid treatments have stimulatory effects on guppy brain aromatase even at concentrations that can be found in the environment. We thus suggest bAA of adult guppies to be a suitable bioindicator of endocrine disruptors.

Fish with thermolabile sex determination (TSD) as models to study brain sex differentiation

As fish are ectothermic animals, water temperature can affect their basic biological processes such as larval development, growth and reproduction. Similar to reptiles, the incubation temperature during early phases of development is capable to modify sex ratios in a large number of fish species. This phenomenon, known as thermolabile sex determination (TSD) was first reported in Menidia menidia, a species belonging to the family Atherinopsidae. Since then, an increasing number of fish have also been found to exhibit TSD. Traditionally, likewise in reptiles, several TSD patterns have been described in fish, however it has been recently postulated that only one, females at low temperatures and males at high temperatures, may represent the "real" or "true" TSD. Many studies regarding the influence of temperature on the final sex ratios have been focused on the expression and activity of gonadal aromatase, the enzyme involved in the conversion of androgens into estrogens and encoded by the cyp19a1a gene. In this regard, teleost fish, may be due to a whole genome duplication event, produce another aromatase enzyme, commonly named brain aromatase, encoded by the cyp19a1b gene. Contrary to what has been described in other vertebrates, fish exhibit very high levels of aromatase activity in the brain and therefore they synthesize high amounts of neuroestrogens. However, its biological significance is still not understood. In addition, the mechanism whereby temperature can induce the development of a testis or an ovary still remains elusive. In this context the present review is aimed to discuss several theories about the possible role of brain aromatase using fish as models. The relevance of brain aromatase and therefore of neuroestrogens as the possible cue for gonadal differentiation is raised. In addition, the possible role of brain aromatase as the way to keep the high levels of neurogenesis in fish is also considered. Several key examples of how teleosts and aromatase regulation can offer more insight into basic mechanisms of TSD are also reviewed.

Fadrozole and finasteride exposures modulate sex steroid- and thyroid hormone-related gene expression in Silurana (Xenopus) tropicalis early larval development

Steroidogenic enzymes and their steroid products play critical roles during gonadal differentiation in amphibians; however their roles during embryogenesis remain unclear. The objective of this study was to investigate the expression and activity of aromatase (cyp19; estrogen synthase) and 5 beta-reductase (srd5 beta; 5 beta-dihydrotestosterone synthase) during amphibian embryogenesis. Expression and activity profiles of cyp19 and srd5 beta were first established during Silurana (Xenopus) tropicalis embryogenesis from Nieuwkoop-Faber (NF) stage 2 (2-cell stage; 1h post-fertilization) to NF stage 46 (beginning of feeding; 72 h post-fertilization). Exposures to fadrozole (an aromatase inhibitor; 0.5, 1.0 and 2.0 microM) and finasteride (a putative 5-reductase inhibitor; 25, 50 and 100 microM) were designed to assess the consequences of inhibiting these enzymes on gene expression in early amphibian larval development. Exposed embryos showed changes in both enzyme activities and sex steroid- and thyroid hormone-related gene expression. Fadrozole treatment inhibited cyp19 activity and increased androgen receptor and thyroid hormone receptor (alpha and beta) mRNAs. Finasteride treatment inhibited srd5 beta (activity and mRNA), decreased cyp19 mRNA and activity levels and increased estrogen receptor alpha mRNA. Both treatments altered the expression of deiodinases (thyroid hormone metabolizing enzymes). We conclude that cyp19 and srd5 beta are active in early embryogenesis and larval development in Silurana tropicalis and their inhibition affected transcription of genes associated with the thyroid and reproductive axes.

Immunochemical and molecular characterization of a novel cell line derived from the brain of Trachinotus blochii (Teleostei, Perciformes): A fish cell line with oligodendrocyte progenitor cell and tanycyte characteristics

Ependymal radial glial cells, also called tanycytes, are the predominant glial fibrillary acidic protein (GFAP)- and vimentin (VIM)-expressing cells in fish ependyma. Radial glial cells have been proposed to be neural stem cells but their molecular expression is not well understood. Previous studies revealed that fish neural progenitor and neural stem cells have A2B5, a marker for oligodendrocyte progenitor cells (OPCs). In this study, an A2B5(+) cell line, SPB, was isolated from the brain of the teleost Trachinotus blochii and characterized. SPB cells usually grew as polygonal epithelial cells, but at high density, long processes were commonly observed. Using immunocytochemistry, SPB cells were shown to exhibit oligodendrocyte markers such as galactocerebroside and Olig2, and radial glial cell markers such as brain lipid-binding protein, GFAP, Sox2, and VIM. SPB cells were also observed to have DARPP-32, a marker for tanycytes in mammals, and primary cilia. RT-PCR additionally revealed expression of bone morphogenetic protein 4, connexin35, Noggin2, and proteolipid protein in SPB cells. Results of this study suggest that SPB cells are OPCs that can display tanycyte characteristics. Fish tanycytes can be neural stem cells suggesting that SPB cells are neural stem cells. SPB is the first fish cell line showing primary cilia and markers for both OPCs and tanycytes.

Early regulation of brain aromatase (cyp19a1b) by estrogen receptors during zebrafish development

Early expression of estrogen receptors (esr) and their role in regulating early expression of cyp19a1b encoding brain aromatase were examined in the brain of zebrafish. Using in toto hybridization and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), a significant increase in the expression of esr1, esr2a, and esr2b was observed between 24 and 48 hours postfertilization (hpf). In toto hybridization demonstrated that esr2a and esr2b, but not esr1, are found in the hypothalamus. Using real-time RT-PCR, an increase in cyp19a1b mRNAs occurs between 24 and 48 hpf, indicating that expression of cyp19a1b is temporally correlated with that of esr. This increase is blocked by the pure anti-estrogen ICI182,780. Furthermore, E2 treatment of cyp19a1b-GFP (green fluorescent protein) transgenic embryos results in appearance of GFP expression in the brain as early as 25 hpf. These results indicate that basal expression of cyp19a1b expression in the brain of developing zebrafish most likely relies upon expression of esr that are fully functional before 25 hpf.

Profiling neuroendocrine gene expression changes following fadrozole-induced estrogen decline in the female goldfish

Teleost fish represent unique models to study the role of neuroestrogens because of the extremely high activity of brain aromatase (AroB; the product of cyp19a1b). Aromatase respectively converts androstenedione and testosterone to estrone and 17β-estradiol (E2). Specific inhibition of aromatase activity by fadrozole has been shown to impair estrogen production and influence neuroendocrine and reproductive functions in fish, amphibians, and rodents. However, very few studies have identified the global transcriptomic response to fadrozole-induced decline of estrogens in a physiological context. In our study, sexually mature prespawning female goldfish were exposed to fadrozole (50 μg/l) in March and April when goldfish have the highest AroB activity and maximal gonadal size. Fadrozole treatment significantly decreased serum E2 levels (4.7 times lower; P = 0.027) and depressed AroB mRNA expression threefold in both the telencephalon ( P = 0.021) and the hypothalamus ( P = 0.006). Microarray expression profiling of the telencephalon identified 98 differentially expressed genes after fadrozole treatment ( q value <0.05). Some of these genes have shown previously to be estrogen responsive in either fish or other species, including rat, mouse, and human. Gene ontology analysis together with functional annotations revealed several regulatory themes for physiological estrogen action in fish brain that include the regulation of calcium signaling pathway and autoregulation of estrogen receptor action. Real-time PCR verified microarray data for decreased (activin-βA) or increased (calmodulin, ornithine decarboxylase 1) mRNA expression. These data have implications for our understanding of estrogen actions in the adult vertebrate brain.

Neuroprotective actions of brain aromatase

The steroidal regulation of vertebrate neuroanatomy and neurophysiology includes a seemingly unending list of brain areas, cellular structures and behaviors modulated by these hormones. Estrogens, in particular have emerged as potent neuromodulators, exerting a range of effects including neuroprotection and perhaps neural repair. In songbirds and mammals, the brain itself appears to be the site of injury-induced estrogen synthesis via the rapid transcription and translation of aromatase (estrogen synthase) in astroglia. This induction seems to occur regardless of the nature and location of primary brain damage. The induced expression of aromatase apparently elevates local estrogen levels enough to interfere with apoptotic pathways, thereby decreasing secondary degeneration and ultimately lessening the extent of damage. There is even evidence suggesting that aromatization may affect injury-induced cytogenesis. Thus, aromatization in the brain appears to confer neuroprotection by an array of mechanisms that involve the deceleration and acceleration of degeneration and repair, respectively. We are only beginning to understand the factors responsible for the injury-induced transcription of aromatase in astroglia. In contrast, much of the manner in which local and circulating estrogens may achieve their neuroprotective effects has been elucidated. However, gaps in our knowledge include issues about the cell-specific regulation of aromatase expression, steroidal influences of aromatization distinct from estrogen formation, and questions about the role of constitutive aromatase in neuroprotection. Here we describe the considerable consensus and some interesting differences in knowledge gained from studies conducted on diverse animal models, experimental paradigms and preparations towards understanding the neuroprotective actions of brain aromatase.

Brain aromatization: classic roles and new perspectives

Aromatization of testosterone to estradiol by neural tissue has classically been associated with the regulation of sexual differentiation, gonadotropin secretion, and copulatory behavior. However, new data indicate that the capacity for aromatization is not restricted to the endocrine brain and demonstrate roles for locally formed estrogens in neurogenesis and in responses of brain tissue to injury. This article summaries our current understanding of the distribution and regulation of aromatase in the brain and describes the classic and novel roles it plays. A better understanding of brain aromatization could shed new light on its physiologic and pathologic functions and someday lead to new, centrally acting drug therapies.

Bisphenol A prevents the synaptogenic response to estradiol in hippocampus and prefrontal cortex of ovariectomized nonhuman primates

Exposure measurements from several countries indicate that humans are routinely exposed to low levels of bisphenol A (BPA), a synthetic xenoestrogen widely used in the production of polycarbonate plastics. There is considerable debate about whether this exposure represents an environmental risk, based on reports that BPA interferes with the development of many organs and that it may alter cognitive functions and mood. Consistent with these reports, we have previously demonstrated that BPA antagonizes spine synapse formation induced by estrogens and testosterone in limbic brain areas of gonadectomized female and male rats. An important limitation of these studies, however, is that they were based on rodent animal models, which may not be representative of the effects of human BPA exposure. To address this issue, we examined the influence of continuous BPA administration, at a daily dose equal to the current U.S. Environmental Protection Agency's reference safe daily limit, on estradiol-induced spine synapse formation in the hippocampus and prefrontal cortex of a nonhuman primate model. Our data indicate that even at this relatively low exposure level, BPA completely abolishes the synaptogenic response to estradiol. Because remodeling of spine synapses may play a critical role in cognition and mood, the ability of BPA to interfere with spine synapse formation has profound implications. This study is the first to demonstrate an adverse effect of BPA on the brain in a nonhuman primate model and further amplifies concerns about the widespread use of BPA in medical equipment, and in food preparation and storage.